As someone who has suffered though inheriting a code base of excessive copy/paste with things done "pretty much the same way but changing only one thing" it's a nightmare to take it to diff and try and refactor out the common code. In my case I had to fix a bug in their copy/paste mess and found six separate copies of a whole functions worth of code duplicated.

I suspect part of the reason is for the initial developer it's a great feeling to churn out tons of code and it all kind of works without having to think how you make a block of code generic (avoiding temptation to just pass in a global state struct and dealing with function pointers in the worst cases etc) but having suffered the after effects, I implore you, PLEASE don't repeat yourself (DRY)

> manually code it into reusable function and use it repeatedly

Refactoring every place where you do x into a reusable function that does x is a boring repetitive chore.